Cantilevered sensing heads are used in Atomic Force Microscopy (AFM) to sense and measure forces between the sensing head and a surface. This generally involves scanning the sensing head across a surface and measuring its deflection. The deflection of the sensing head may be used to investigate the topography of the surface. It can also be used to measure forces acting on the sensing head such as forces exerted on the tip by the surface, electrostatic forces and capillary forces. In order to measure such forces using this technique, it is necessary to know the spring constant of the cantilevered sensing head. Inaccuracies in determination of the spring constant can lead to unacceptable errors in the resulting force measurement.
The spring constant of cantilevered sensing heads can be measured by a variety of methods including theoretical calculation, applying a known mass to statically deflect the cantilever head, deflecting the cantilevered head with another cantilevered head having a known spring constant and by determination of the resonant frequency either in vacuum or in a static pool of liquid. A summary of prior art calibration methods is given in “Calibration of Atomic Force Microscopy Cantilevers” by J E Sader in “Encyclopaedia of Surface and Colloid Science”, published by Marcel Dekker Inc New York [2002] page 846.
These prior art methods have varying problems. For example, where a force is applied using a calibrated weight or another cantilever, the tip of the cantilever may be damaged. This is particularly disadvantageous when a coated cantilevered probe head is used. This applies, for example, to probes having a bio-film coating such as those often used in AFM for biomedical research. Further probe degradation can occur when the probe cannot be calibrated in-situ and has to be moved, calibrated and re-installed.
Another disadvantage of certain known methods is that they are usable only with specific cantilever configurations, such as only for rectangular cantilever heads. Other disadvantages of prior art methods include that they are complicated or difficult to perform or result in inaccuracies due to assumptions made or involve the need to accurately measure other properties such as the mass, volume, profile or density of the cantilever. Further inaccuracies may be introduced by the spring constant being measured in inappropriate medium such as air or a vacuum.